1. Field of the Invention
The present invention relates to a switch for use in electric power, and in particular to a switch which can extinguish an arc generating at an opening time of a contact by using a buffer apparatus to spray an arc-extinguishing gas.
2. Description of the Prior Art
FIG. 1 is a side sectional view showing a state in the course of opening of a conventional switch disclosed in Japanese Utility Model Application Laid-Open No. 59-77741. In FIG. 1, reference numeral 1 means an upper terminal, 2 means a fixed contact attached to the upper terminal 1, 3 is a moving contact which is slid with respect to the fixed contact 2 to make and break contact therewith, 4 is an outer cylinder whose one end is secured to the upper terminal 1, and 5 is a first insulating nozzle secured to one end of the outer cylinder 4, and having a through-hole into which the moving contact is insertable at a closing time. Further, reference numeral 6 means a lower terminal, 7 means a holder formed in the fixed contact 2, 8 is a first permanent magnet mounted to the holder 7, 9 is a pressure accumulator which is surrounded by the upper terminal 1, the outer cylinder 4, and the insulating nozzle 5, and 10 is an arc generating when the moving contact 3 is opened apart from the fixed contact 2.
A description will now be given of the operation. When case current flows from the upper terminal 1 to the fixed contact 2, and to the moving contact 3 in an ON state of the switch, the moving contact 3 is driven in a direction of the arrow A by a drive mechanism (not shown). Accordingly, the fixed contact 2 and the moving contact 3 are opened apart from one another to generate the arc 10 between the contacts. A gas in the pressure accumulator 9 is heated by the arc 10 when a current phase is in a vicinity of a current peak, and pressure in the pressure accumulator increases due to expansion of the gas and molecular decomposition. Further, when the current phase is in a vicinity of a current zero point, a column diameter of the arc 10 becomes thinner, and a temperature thereof decreases. As a result, the gas is inversely sprayed on the arc 10 from the pressure accumulator 9 to extinguish the arc in the vicinity of the current zero point.
However, if the arc current has a small effective value, the arc energy is small. Consequently, a rise of gas pressure in the pressure accumulator 9 is reduced so that a force to spray the gas on the arc 10 becomes weak, resulting in a reduced arc-extinguishing performance. Hence, in the prior art switch, the annular magnet 8 is polarized in a direction of magnetic flux (.phi.) as shown in FIG. 1, and is mounted in the holder 7 so as to generate a magnetic field having radial components in a vicinity of a distal end of the fixed contact 2. The arc 10 in the vicinity of the distal end of the fixed contact 2 is stretched in the pressure accumulator 9 while being driven in a direction of the Lorentz's force (i.e., in a circumferential direction) by the magnetic field having the radial components. Therefore, the gas in the pressure accumulator 9 is efficiently heated by the magnetically impelled arc 10 so that the rise of pressure in the pressure accumulator 9 increases, and the force to spray the gas on the arc 10 increases when the current phase is in the vicinity of the current zero point. Further, the arc 10 is driven to rotate so as to add a relative gas stream between the gas in the pressure accumulator 9 and the arc 10, resulting in an excellent arc-extinguishing performance even if cutoff current is small.
In the prior art switch as set forth above, the pressure accumulator is heated by the arc to increase the pressure thereof, and the arc-extinguishing gas therein flows out of the pressure accumulator in cutoff process. Accordingly, the mass of the arc-extinguishing gas in the pressure accumulator decreases, and the arc-extinguishing gas in the pressure accumulator is heated by the arc so as to gradually become a hot and lower density gas. As a result, there is a problem in that an insulation recovering characteristic is reduced after the current is cutoff.